The subject invention relates generally to attenuator circuitry and particularly to variable attenuator devices employing field effect technology.
Present day electronic applications often require the utilization of attenuator circuits or components which are responsive to control signals to vary the amount of resistance provided thereby. Such attenuators are useful for automatic gain control circuits, position locating systems, telephone systems, television systems, etc.
Prior art, electronically variable solid state attenuators for use at Radio Frequencies (RF) typically employ PIN diodes. PIN diode attenuators may be arranged in a variety of network configurations. Biasing control circuitry which include Field Effect Transistors (FETs) have been utilized to bias the PIN diodes in response to various analog control signal magnitudes, thereby causing the PIN diode network to provide any one of a variety of resistance magnitudes. Discrete PIN diode attenuators are capable of outstanding performance but require an undesirable amount of electrical power for some applications. Moreover, PIN diodes are not easily integrated into monolithic circuitry.
Other prior art approaches for providing electronically variable attenuators sometimes utilize FETs such as gallium arsenide (Ga As) Metal Semiconductor Field Effect Transistors (MESFETs). These devices also may be arranged in a variety of networks and each device may operate without bias and therefore consume almost no electrical power except during switching operations. Analog control signals are applied to the gates of these devices to adjust the attenuation level. The resistance magnitude provided by each FET is adjusted by controlling the depth of the undepleted channel in the device in accordance with the magnitude of the analog signal. Unfortunately, such variation of the depth of the undepleted channel tends to result in a nonlinear transfer characteristic which provides high intermodulation levels between RF signals applied to these devices. This causes the generation of unwanted frequency components that result in distortion.
To minimize such distortion, some prior art attenuators utilize additional circuitry which undesirably increases cost, size, weight and undesirably decreases reliability. Moreover, prior art attenuators sometimes require precise control of the magnitudes of the analog control signals. Such precision is difficult to achieve because analog signal magnitudes tend to drift with temperature, age of the semiconductors, variations of process parameters from device to device, etc. Also, some MESFET attenuator networks tend to have a limited tuning range.
Monolithic Microwave Integrated Circuits (MMIC) applications using GaAs semiconductor material are presently being developed because of the high frequency handling capabilities and the small size of such circuits. These applications require electronically variable attenuators which are compatible with the presently available MMIC semiconductor fabricating processes. The aforementioned prior art attenuators tend to be too expensive to fabricate, take too much space and/or have an unduly high failure rate for some of these applications. Also, some complex prior art attenuators utilizing analog control signals tend to operate too slowly to take advantage of the inherent speed characteristics of MMIC circuitry.